Arc ionizable beryllium electrodes for vacuum arc devices



July 7, 1964 F. H. HO'RN 3,140,373

ARC IONIZABLE BERYLLIUM ELECTRODES FOR VACUUM ARC DEVICES Filed Jan. 24, 1962 2 Sheets-Sheet 1 I III/III) 'I/I/I/I/I/ In ventor: Foryce. Nor-r1, by I F is A ttroz-ney.

July 7, 1964 F. H. HORN 3,140,373

ARC IONIZABLE BERYLLIUM ELECTRODES FOR VACUUM ARC DEVICES Filed Jan. 24. 1962 2 Sheets-Sheet 2 CUkkEA/T 0 lo z'o 3.0 4.0 5.0

GAP 451/67 mm Inventor:

is Attorney.

United States Patent 3,140,373 ARC IONIZABLE BERYLLIUM ELECTRODES FOR VACUUM ARC DEVICES Fordyce H. Horn, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 24, 1962, Ser. No. 168,436 6 Claims. (Cl. 200-144) The present invention relates to vacuum are devices. More particularly, the invention relates to vacuum are devices which exhibit extremely high static breakdown voltages and rapid recovery times.

Vacuum arc devices are becoming increasingly more important in the control of high current, high voltage electric energy. One such device is the vacuum switch or vacuum circuit interrupter, which is operative to interrupt currents of thousands of amperes in the kilovolt ranges, and, in the case of an alternating current voltage, to cause extinction thereof upon the occurrence of a first current zero. Devices of this type are set forth and claimed in US. Patents Number 2,975,255--Laiferty and 2,975,256-Lee et al.

Other such devices include lightning arresters and vacuum gaps. Lighting arresters and vacuum gaps are vacuum devices with fixed electrodes designed to withstand high electric stress up to a point, but to break down and conduct electricity upon the occurrence of a given overload and to continue conduction until a transient overload has passed or until the occurrence of a next current zero whichever is sooner.

In both of the aforementioned types of vacuum arc devices, it is essential that the device be able to clear itself and be ready to withstand a high voltage without breaking down shortly after extinction of a high current arc. The time within which such a recovery is effected is known as the recovery time. The degree to which recovery is efiected within a given time is known as re covery strength. In both vacuum switches and vacuum gap devices it is desirable that, once recovery has been effected, the device exhibits high breakdown strength. This characteristic is particularly desirous and necessary in vacuum gap devices which normally operate with an open gap between the electrodes, as opposed to vacuum switches which normally operate with no gap between the electrodes. Thus, in the operation of a vacuum gap device it is, necessary that high voltages be withstood before breaking down.

Accordingly, it is one object of the present invention to provide improved vacuum gap devices of the type described.

Another object of the present invention is to provide vacuum are devices having high static breakdown strength, rapid recovery time and high recovery voltage. Yet another object of the present invention is to provide improved vacuum switches having'rapid recovery times.

Still another object of the invention is to provide vacuum gap devices having rapid recovery strength and unusually high static breakdown voltages.

In accord with the present invention, vacuum are. devices are provided in the form of evacuable envelopes or chambers capable of maintaining a vacuum of less than millimeters of mercury, which chambers contain a pair of arc electrodes adapted to be the terminal points of 'an electric are carrying an electric curnent. In one embodiment of the invention one are electrode is adapted to be movable and to abutt against the other, thus facilitating the interruption of electric currents. In another embodiment of the invention both electrodes are fixed and are spaced apart from one another to define a vacuum gap therebetween. In both embodiments of the invention the portions of the arc electrodes which serve as terminal points of an electric arc are substanice tially free of occluded and absorbed gasesand comprise beryllium, either in the elemental state or as an alloy with another electrode material, as for example, copper or silver.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention, however, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the attached drawing in which:

FIGURE 1 is a vertical cross-sectional view of a representative vacuum circuit interrupter in accord With one embodiment of the invention, and

FIGURE 2 is a vertical cross-sectional view of a representative vacuum gap device constructed in accord with another embodiment of the present invention.

In FIGURE 1, an interrupter chamber 10 comprises a wall member 11 which may be cylindrical in shape and is constructed of a suitable insulating material, having at the ends thereof a pair of metallic end members 12 and 13 enclosing the volume therein to form an interrupter chamber. Suitable shields 14 are provided between casing 11 and end members 12 and 13 to render the interrupter chamber vacuum tight.

Located within chamber 10 are a pair of separable contacts or electrodes 15 and 16 shown in their closed circuit or engaged position. Upper contact 15 is a stationary contact suitably attached electrically and mechanically to a conducting rod 17 which, at its upper end, is united electrically and mechanically with end member 12. Lower contact 18, mounted upon and electrically united with a suitable conducting rod 18, is movable and is connected through bellows 20 or an equivalent vacuum tight member, permitting reciprocating motion. Termi nal mounting rod 18 projects through a suitable orifice in end member 13, and suitable actuating means may be connected thereto to cause a reciprocating motion of rod 18 to cause contact 16 to enter into engagement with, and to be removed out of engagement from, contacts 15. The electrical circuit which is sought to be interrupted by the interrupter device may be completed by making suitable connections to terminal 21, electrically and mechanically mounted upon end member 12, and terminal 22, electrically and mechanically mounted upon rod 18. A suitable insulator shield, such as metallic cylindrical member 23,'capped with anarc preventing ferrule 24, is

interposed between electrodes 15-16 and insulator 11 to prevent the latter from becoming coated with metal and becoming electrically short-circuited.

The volume within vacuum interrupter chamber 10' is suitably evacuated through an exhaust tubulation (not shown) during the final assembly thereof. For proper operation of the interrupter as a vacuum type interrupter of alternating currents, the pressure within chamber 10 must be maintained at a pressure no greater than 10' millimeters of mercury and preferably a pressure less than 10' millimeters of mercury. The foregoing requirement is essential for the operation of the devices as vacuum interrupters of alternating currents. This requirement is necessary because, in order that a currentcarryingarc struck between electrodes 15 and 16 be extinguished at the first occurring current zero. value, there must'be substantially no ionizable gas presentwithin chamber 10. The occurrence of such ionization may be substantially prevented if the possible breakdown paths between electrodes 15 and 16, or their respective supports, are small with respect to the mean free path of an electron within the atmosphere obtained within the device. This mean free path is designated a statistical distance which an electron may travel without colliding with a gas molecule at a given pressure. Theseconditions may be established within the devices of the pres ent invention only when the pressure within the interrupter chamber 10 is below 10 millimeters of mercury and preferably when it is below l6 millimeters of mercury.

One type of commercially available vacuum circuit interrupter utilizes contacts of tungsten, molybdenum or other refractory materials which may be baked and otherwiseheat treated for a sufficiently long time and at a sufliciently high temperature to remove all absorbed gases from the vacuum arc. This is necessary because, the vacuum arc is sustained as a conducting column by ionized metallic particles which are boiled from the electrodes, primarily the cathode. During the boiling out process, if any absorbed gases or gas-forming constituents are present within the electrode material, gas is freed and becomes ionized. Upon the occurrence of the first current zeroafter establishment of an arc, the ionized metallic vapors within the chamber rapidly cool and migrate back to the electrodes, to the shield, or to the chamber wall, are de-ionized and removed from the chamber, lowering the pressure. This is not, however, the case with ionizable gases which are boiled from the electrodes. Such gases continue to remain in the chamber and raise the pressure thereof. It is for this reason that the electrodes must be purified to such an extent as to cause the presence of gas and gas-forming constituents Within the electrodes to be low enough to allow the maintenance of a vacuum of less than 10* millimeters of mercury pressure within the device.

It is, of course, necessary to provide a standard for occluded and absorbed gases for vacuum interrupter contacts in order that interrupters be operative. Such a standard is provided by the requirement that, for use in the devices of the present invention, contacts, when placed in a vacuumized test chamber a few liters in volume and subsequently deeply erroded by a repetitive arcing, as for example, with a voltage of commercial power at a current of 100 amperes or more, the pressure level of the container, a few cycles after arcing, does not rise substantially from its initial value in the absence of geters and pumps even if the initial pressure is of the order of millimeters of mercury. Analytically, this requirement may be stated in the relationship that the contact material must contain less than 10 atomic parts of all gases or gas-forming constituents. Since arc errosion in accord with the aforementioned test consumes a substantial portion of the contact material, it proves that it provides a satisfactory test for the determination whether the arc electrodes are substantially free of all occluded and absorbed gases, since even infinitesimal presence of such would cause a substantial rise in the presence of the test chamber.

Vacuum switches, as described above, constructed utilizing refractory type electrodes which may be readily provided in a suflicient gas-free-state suffer from a serious difficulty in that their current chopping characteristic is very bad. Current chopping, a term well known to the vacuum circuit interrupter art, is characterized by the instantaneous and premature extinction of the arc prior to the arrival of a current zero upon interruption of an alternating current circuit. Current chopping of an alternating current is shown graphically by the curve of FIG- URE 2, in which sinusoidal current wave A is interrupted at time Ti and the arc chops at time Tc, creating a transient current change Io. This event occurs at some finite value of current and, upon the occurrence thereof, there is a sudden change in the current flowing in the circuit of the order of the chopping current. In inductive circuits, this transient current change causes high transient voltages which can ruin expensive equipment. While a precise chopping current may not be specified for any given material, statistical studies make it possible to determine a current at which the probability of instantaneous and interruption of the alternating current are may occur. This current is generally accepted as the chopping current for any given material. For tungsten and molybdenum chopping currents of the order of 15 amperes are not unusual.

As a solution to high chopping current problems, it has been discovered and is described and claimed in US. Patents Number 2,975,255-Lafferty and 2,975,256 Lee et al., that certain high vapor pressure elements and alloys may be utilized to cause a plenteous supply of metallic ions to be present within the vacuum interrupter chamber during interruption, even at low currents, thus facilitating the maintenance of an arc in operation until a very low current is obtained. In accord with one aspect of the invention chopping currents as low as 4 amperes and lower are obtained utilizing alloys of copper and bismuth and the like. These materials introduce some problems of processing, since vacuum switches utilizing electrodes of these materials may not be baked for long times at high temperatures, as may vacuum switches utilizing refractory electrodes. Other processing may, however, be utilized as set forth in the aforementioned patents, to obtain gas-free contacts of these improved materials which exhibit low chopping characteristics.

Even with the aforementioned low chopping contacts, it has been found that recovery times are often quite long. It is believed that low recovery times of low chopping current electrode containing vacuum switches are due at least in part to the fact that the high concentration of ionized metallic particles in the vicinity of the arc at low currents causes a delay of a finite time during which these materials must migrate to a cold wall in order to condense thereupon and be removed from the vacuum chamber. Upon a careful analysis of all the data relevant to this phenomenon, it has been noted that this finite time is greater with higher molecular weight materials.

In accord with the present invention, I provide vacuum are devices including alternating current circuit interrupters utilizing electrodes composed in whole or in part of beryllium. Beryllium is effective in solving the recovery time problem in that beryllium, with an atomic number of 4 and an atomic mass of 9, is an extremely light material the ions of which appear to rapidly difiuse to cause the attainment of rapid recovery times in vacuum arc devices in which such electrodes are utilized. In addition to the foregoing, it is found that the chopping current obtainable from devices utilizing beryllium electrodes is approximately of the same order of that obtainable utilizing copper, namely approximately 4 to 6 amperes.

To study the improvement which may be achieved in recovery time and recovery strength of vacuum are devices by the use of beryllium, comparison with other contact materials were made. When zinc, a high vapor pressure material which may be utilized to overcome chopping in vacuum interrupters is utilized as the contact material at currents higher than 400 amperes, a drastically reduced recovery strength is observed at any time less than 15 microseconds after arcing. When copper, a material which is not quite a good as zinc from the chopping point of view, but which is better from the recovery strength point of view is utilized, and an arc struck across a gap at a current of 1600 amperes, the gap recovers to a strength of 40 kilovolts after a period of 10 microseconds. Using beryllium, on the other hand, at the same current, the same gap has, after 10 microseconds recovered to 'a strength of 65 kilovolts, an improvement of approximately 60% in recovery strength.

A further unique advantage obtainable from vacuum are devices, constructed in accord with the present invention is the attainment of high static breakdown voltage. In FIGURE 4 of the drawing there are plotted the static breakdown voltage curves as a function of gap length for electrode pairs of zinc (curve A), silver (curve B), an intermetallic compound of Cu Sn (curve C), an alloy of 20% bismuth the remainder copper (curve D), an

alloy of copper, tungsten and thorium (curve E) and beryllium (curve F). As may readily be observedfrom FIGURE 2, the static breakdown of contacts of beryl lium, as a function of gap length is vastly superior to any other material considered. At a representative gap length of 1 millimeter the recovery strength of zinc is approximately 33 kilovolts; that of silver is approximately 58 kilovolts; that of copper is approximately 50 kilovolts; and that of beryllium is approximately 95 kilovolts. This characteristic is especially useful in vacuum gap devices.

A vacuum gap device constructed in accord with the present invention is illustrated in FIGURE 3 of the drawing. In FIGURE 3 gap chamber 30 comprises a wall member 31 which may be cylindrical in shape and is constructed of a suitable insulating material having at the ends thereof a pair of metallic end members 32 and 33 closing the volume therein to form a vacuum chamber. Suitable shields 34 are provided between casing 31' and end members 32 and 33 to render the chamber vacuum tight. Located within chamber 30 are a pair of spaced contacts or gap electrodes 35 and 36 which define a vacuum gap 37 therebetween. Upper contact 35-is suitably attached electrically and mechanically to a conducting rod 38 which is at its upper end united electrically and mechanically with end wall member 32. Lower contact 36 is mounted upon and electrically united with conducting rod 39 which is electrically and mechanically connected with end wall member 33. The electrical circuit which is sought to be protected or connected to vacuum gap device 30 is connected to end Wall members 32 and 33 by means of connecting lugs 40 and 41, electrically and mechanically affixed to respective wall end members 32 and 33. A suitable insulator shield such as metallic cylindrical member 43 capped with an arc-preventing ferrule 44 is interposed between electrodes 35-36 and insulator 31 to prevent the latter from becoming coated with metal and becoming electrically short-circuited. The volume within chamber 30 'is maintained at a pressure of less than and preferably less than 1O millimeters of mercury as with respect to the device of FIGURE 1.

Contacts 35 and 36 are constructed in whole or in part of beryllium which is properly treated as with respect to the contacts and 16 of the device of FIGURE 1 in order to provide freedom from gas or gas-containing compounds in order to maintain the high vacuum conditions within chamber 30 as is described with respect to the device of FIGURE 1. The devices constructed in accord with FIGURES 1 and 3 of the invention possess the rapid recovery time, high recovery strength and the static breakdown strength attributable to beryllium. While, to optimize these characteristics, it is desirable to have as high a proportion of beryllium as is possible in contacts utilized in the devices, it may be advisable that the beryllium be admixed in an alloy of another material in order that suitable mechanical strength, anti-welding characteristics, ductility and long life may be obtained. To this extent the contacts may comprise an alloy of beryllium with approximately as much as 50% by Weight copper or silver, although it is preferable that no more than by Weight copper or silver be utilized. Alternatively, substantially pure beryllium or an alloy of copper and beryllium or silver and beryllium, as set forth hereinbefore, may be purified so as to have less than 10* atomic parts of gas or gas-forming constituents therein and caused to be flowed into a porous matrix of a refractory material which may be formed by sintering tungsten, molybdenum, tungsten-carbide, molybdenum-carbide or like materials. The tungsten or other refractory material contributes primarily the mechanical strength while the interrupting characteristics of the vacuum arc device are supplied by beryllium or the beryllium alloy which is denominated herein as the active arc-sustaining ionizable metallic substance.

The requisite freedom from gaseous impurities and gaseous-forming impurities as set forth herein may readily be obtained in beryllium contacts or in contacts of a beryllium alloy as utilized in the embodiments of the invention by, prior to casting the electrodes, subjecting the beryllium to a directional cooling and crystallization process so controlled as to cause a selective segregation of the gaseous-forming impurities and gases from the crystallized beryllium. Subsequent manipulative steps performed upon the beryllium involving melting, heat treating or other heat-involving process must thereafter be conducted in vacuo to maintain the necessary freedom from gases, principally oxygen.

While the invention has. been set forth herein with respect to certain embodiments thereof many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A vacuum arc device exhibiting rapid recovery time and high static breakdown voltage characteristics and consisting essentially of: an evacuable envelope evacuated to a pressure lower than 10 millimeters of mercury; a pair of electrical terminals adapted for connection in an electrical circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals and adapted to sustain an electric are therebetween, said are being comprised of ionized metallic parts which originate from said electrodes; each of said electrodes having a region upon which the respective elec trode spots for a current-carrying arc may be established and from which said metallic parts originate, at least one of said electrode regions being comprised of an active ionizable arc-sustaining metallic substance consisting essentially of beryllium.

2. A vacuum arc device exhibiting rapid recovery time and high static breakdown voltage characteristics and comprising: an evacuable envelope evacuated to a pressure lower than 10 millimeters of mercury; a pair of electrical terminals adapted for connection in an electrical circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals and adapted to sustain an electric arc therebetween, said arc being comprised of ionized metallic parts which originate from said electrodes; each of said electrodes having a region upon which the respective electrode spots for a current-carrying arc may be established and from which said metallic parts originate, at least one of said electrode regions being comprised of an active ionizable arc-sustaining metallic substance comprising at least weight percent beryllium.

3. A vacuum alternating current circuit interrupter comprising an evacuable envelope evacuated to a pressure lower than 10* millimeters of mercury; a pair of electrical terminals adapted for connection in an alternating current circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals and disposed in spaced apart relationship during circuit interrupting operation to allow for the establishment of a circuit interrupting arc therebetween, said arc being comprised of ionized metallic parts which originate from said electrodes; each of said electrodes having a region upon which the respective electrode spots for a currentcarrying arc may be established and from which said metallic parts originate, said electrode regions being comprised of an active ionizable arc-sustaining metallic substance consisting essentially of beryllium.

4. A vacuum alternating current circuit interrupter comprising an evacuable envelope evacuated to a pres sure lower than 10* millimeters of mercury; a pair of electrical terminals adapted for connection in an alternating current circuit; a pair of electrodes located within said envelope, connected in circuit between said terminals and disposed in spaced apart relationship during interrupting operation to allow for the establishment of a circuit interrupting arc therebetween, said are being comprised of ionized metallic parts which originate from said electrodes; each of said electrodes having a region 7 upon which the respective electrode spotsfor a currentcarrying arc may be established and from which said metallic parts originate, at least one of said electrode regions being comprised of an active ionizable arc-sustaining metallic substance comprising at least 80 weight percent beryllium.

5. A vacuum gap device comprising an evacuable envelope evacuated to a pressure lower than 10- millimetersof mercury; a pair of fixed electrical terminals adapted for connection in an electric circuit; a pair of fixed electrodes located within said envelope, connected in circuit between said terminals and disposed in spaced apart relationship to define a vacuum gap therebetween to allow for the establishment of an electric arc during operation, said are being comprised of ionized metallic particles originating from said electrodes; each of said electrodes having a region upon which respective electrode spots for the electric arc may be established and from which the arc-sustaining particles are taken, at least one of said electrode regions comprising an active ionizable arc-sustaining metallic substance consisting essentially of beryllium.

6. A vacuum gap device comprising an evacuable envelope evacuated to a pressure lower than 10- millimeters of mercury; a pair of fixed electrical terminals adapted for connection in an electric circuit; a pair of fixed electrodes located within said envelope, connected in circuit between said terminals and disposed in spaced apart relationship to define a'vacuum gap therebetween to allow for the establishment of an electric are therebetween during operation, said are being comprised of ionized particles of ionized metallic particles originating from said electrodes; each of said electrodes having a region upon which the respective electrode spots for the electric arc may be established and from which the arcsustaining particles are taken, at least one of said electrode regions being comprised of an active arc-sustaining ionizable metallic substance comprising at least 80 weight percent beryllium.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A VACUUM ARC DEVICE EXHIBITING RAPID RECOVERY TIME AND HIGH STATIC BREAKDOWN VOLTAGE CHARACTERISTICS AND CONSISTING ESSENTIALLY OF: AN EVACUABLE ENVELOPE EVACUATED TO PRESSURE LOWER THAN 10**-5 MILLIMETERS OF MERCURY; A PAIR OF ELECTRICALL TERMINAL ADAPTED FOR CONNECTION IN AN ELECTRICAL CIRCUIT; A PAIR OF ELECTRODES LOCATED WITHIN SAID ENVELOPE, CONNECTED IN CIRCUIT BETWEEN SAID TERMINALS AND ADAPTED TO SUSTAIN AN ELECTRIC ARC THEREBETWEEN, SAID AR BEING COMPRISED OF IONIZED METALLIC PARTS WHICH ORIGINATE FROM SAID ELECTRODES; EACH OF SAID ELECTRODES HAVING A REGION UPON WHICH THE RESPECTIVE ELECTRODE SPOTS FOR A CURRENT-CARRYING ARC MAY BE ESTABLISHED AND FROM WHICH SAID METALLIC PARTS ORIGINATE, AT LEAST ONE OF SAID ELECTRODE REGIONS BEING COMPOSED OF AN ACTAIVE IONIZABLE ARC-SUSTAINING METALLIC SUBSTANCE CONSISTING ESSENTIALLY OF BERYLLIUM. 